This invention relates to planar phased arrays used in source location, source imaging or target illumination applications, and more particularly to a multi-arm, elliptic logarithmic spiral array for providing reduced sidelobe contamination, particularly in applications where off-axis beamforming is required.
Phased arrays, and particularly aeroacoustic phased arrays, have become a standard measurement tool for noise engineering. Such phased arrays are frequently used in development tests of various products such as aircraft, and employed in wind tunnels to enable simultaneous aerodynamic and acoustic data acquisition.
The present invention is directed to the problem of designing a planar phased array which is useful across a broad range of frequencies, and where the available number of sensors in the array is restricted such that a regular (i.e., equally spaced element) array cannot be achieved with intra-sensor spacing meeting the half-wavelength criteria typically required to avoid spatial aliasing contamination in source maps or projected beams. A particular problem for such planar arrays is where the primary direction for beamforming is substantially off-axis of the array. This is an especially common problem, for example, for aeroacoustic phased array measurements taken in wind tunnels and fly-over noise measurements. When the phased array is used within a wind tunnel it is commonly placed flush in the wall of the wind tunnel or flat on the ground so that the array orientation is restricted. In such an application, the primary xe2x80x9clookxe2x80x9d direction will be determined by the position of the model under test with respect to the array position in the wall of the wind tunnel. Beamforming must then be performed off-axis, which reduces the effective aperture of the array. In particular, circular arrays are less effective in beamforming in the off- axis direction and suffer a loss of resolution in the dimension corresponding to the look direction relative to the resolution in the direction perpendicular to the look direction.
It is therefore a principal object of the present invention to provide a planar array that is particularly well adapted to be used in aerocoustic applications where off-axis beamforming is required. More specifically, it is a principal object of the present invention to provide a planar array which is especially well suited to performing off-axis beamforming without suffering reduced resolution in the look direction typically experienced with circular arrays in such applications.
The above and other objects are provided by a multi-arm, elliptic logarithmic spiral array in accordance with a preferred embodiment of the present invention. The array is formed by first producing an elliptic logarithmic spiral. Next, the elliptic logarithmic spiral is sampled by any one of a number of methods to provide a plurality of sample points angularly spaced apart thereon at which sensors are located. An ellipse is then formed off of each sample point on the elliptic logarithmic spiral such that each ellipse has the same eccentricity as an ellipse that is used to determine a maximum radius of the elliptic logarithmic spiral. All of the ellipses are further formed such that they are concentric with one another.
Finally, each ellipse is sampled with an odd number of equi-angularly spaced samples over a 2 TT angle. Sensors are then placed at each of the sample points. The sampling of each ellipse further begins at that point where the elliptic logarithmic spiral crosses the given ellipse.
The multi-arm, elliptic logarithmic spiral array of the present invention is non-redundant, meaning that no vector spacing between any two sample points (i.e., elements) in the array is repeated. The array of the present invention produces excellent side lobe characteristics over a broad range of frequencies.
The multi-arm, elliptic logarithmic spiral array of the present invention is especially well suited to aerocoustic applications where the primary application is off-axis beamforming. The ellipses of the array are orientated such that their major axes extend along a primary look direction, which is determined by the position of the model under test with respect to the array position. The minor axes of the ellipses are then disposed perpendicular to the look direction. When sensor elements are positioned at the sampling points on each of the ellipses, the array is able to perform off-axis beamforming without the typical reduction in aperture size that occurs with conventional circular logarithmic arrays.